1. Field of the Invention
This invention relates to scanners, and more particularly, to a motor-speed control apparatus for use on a scanner of the type having a movable CCD (charge coupled device) scan head, for the purpose of controlling the operation of a stepper motor used to move the scan head in such a manner as to allow the scanner to perform the scanning process more efficiently.
2. Description of Related Art
A CCD (charge coupled device) is a semiconductor device with a grid of light-sensitive elements capable of converting light images into electrical signals. Fundamentally, the magnitude of the electrical signal generated by a CCD is proportional to the intensity of the light illuminated on the light-sensitive surface of the CCD. In practice, a number of CCD elements are arranged into an array to form a CCD module that can be used in a wide range of applications. For instance, the scan head of a scanner uses a CCD module to convert the light image of a document into electrical signals. A CCD scan head is typically composed of three CCD sensors, respectively used to detect the red (R), green (G), and blue (B) components of the reflected light from the document being scanned. The RGB signals generated by the CCD module can then be further processed to obtain the digital image of the scanned document. The three RGB sensors are activated by respective trigger signals during the scanning process.
Scanners typically come with various resolutions, such as 600 dpi (dot per inch), 300 dpi, and 200 dpi. The user can select a desired resolution from these options for the scanner to scan the document accordingly. A high-resolution digital image contains a greater number of pixels than a low-resolution image. Therefore, when a low resolution setting is specified by the user to display a high-resolution image, some pixels in the high-resolution image should be discarded. For example, 400 dpi to display an originally high-resolution image of 600 dpi, the 1st, 2nd, 3rd, 4th, 5th, and 6th pixels in the same line of the high-resolution image may be processed in such a manner that the 2nd and the 5th pixels are discarded, and only the 1st, 3rd, 4th, and 6th pixels are retained in the low-resolution version of the original image. In the case of setting a resolution of 200 dpi for the same 600 dpi image, the pixels may be processed in such a manner that the 2nd, 3rd, 5th, and 6th pixels are discarded and only the 1st and 4th are retained in the low-resolution version of the original image. One problem of lowering the resolution, however is that some image details in the original high-resolution image are lost when it is displayed at a low resolution. It is therefore an important research topic in the industry of imaging technology to find ways to retain the details of the original high-resolution image as much as possible when the original is displayed at a lower resolution.
Conventionally, there are many solutions to the foregoing problems, which include a software method and a hardware method.
In the software method, a software program is used to determine which of the CCD elements in the scan head are to be activated based on the current resolution setting. These CCD elements are then activated in the prescribed manner during the scanning process. One drawback to this method, however, is that the involved software computation is relatively time-consuming, which makes the operation of the scanner process quite inefficient. Moreover, the software method does not allow the stepper motor used to drive the CCD scan head to maintain a fixed speed during the scanning process.
By the hardware method, a memory unit is used to store the originally obtained digital data of the scanned image. In the case that the scanned image is to be converted to a lower resolution version, the original data are processed to discard some pixels in the original image. One drawback to this method, however, is that its implementation requires a large amount of memory space since the original high-resolution image typically has a very large amount of data. For this reason, the hardware cost to implement this method is quite high and results in a scanner that is less competitive in the market. As a summary, the conventional methods for converting a high-resolution scanned image to a low-resolution version have the following disadvantages:
(1) First, in the software method, the scanning process is considerably inefficient due to the fact that a software computation process must be performed to determine which of the CCD elements in the scan head are to be activated during the scanning process.
(2) Second, in the software method, the stepper motor for moving the CCD scan head over the scanned document is unable to maintained a constant, fixed speed during the scanning process, which can vibrate the scanner and thus degrade the quality of the scanned image.
(3) Third, in the hardware method, the implementation cost is high due to the fact that a large amount of memory space is required to store the data of the originally obtained, high-resolution, scanned image.
(4) Fourth, since the amount of data of the original high-resolution image is quite large, a great deal of time is needed to process and transfer these data in the scanner, resulting in degradation of the operating efficiency of the scanner.